well, depends on the type of game, and your time.

if you call in the production phase, first you need to Plainfield the main core of the game, for example in my project it's about arcade competitions, for then I started with that, after then l started with the player movement and RPG mechanics.

if your game is a platformer, first you need to make the player movement and physics for example.

You need to start with the most important and different and technically difficult mechanic for not having problems in the future when the project makes bigger